You can select Market-on-Close (MOC) or Limit-on-Close (LOC) as the order type for a One Cancels All (OCA) group or for a Profit taking leg attached to a Parent order. Your Market-on-Close or Limit-on-Close order is given to the NYSE at 15:45 EST if the NYSE is the Primary exchange or to the Nasdaq (Island) at 15:50 EST if the Nasdaq is the Primary exchange. After that time you will be unable to Modify or Cancel that MOC or LOC order.
It is important to note that it would be possible for the working bracket order or OCA Stop leg to execute in the last 10 or 15 minutes of the regular trading session and close an open position. Subsequently, when IBKR sends a cancel request to the exchange holding your OCA leg for the MOC or LOC order the cancel request would not be acted upon by the exchange. In this case, your MOC/LOC order being held by the exchange could still execute and open a new position.
For more information, please visit the Interactive Brokers website pages for Market-on-Close and Limit-on-Close orders.